A typical example of an integrated circuit with components of two somewhat different types is a non-volatile memory (an EPROM, an EEPROM or a "flash" memory) with the respective control circuits, that is, the reading, writing and, in the case of "flash" memories and EEPROMs, erasing circuits.
As is known, the cells of a non-volatile memory comprise, in their structures, a "floating" gate electrode separated from the silicon surface by a dielectric. The floating gate electrode is usually constituted by a thin, doped polycrystalline element and the dielectric is a thin layer (6-35 nm) of silicon dioxide produced by oxidation of the silicon at high temperature (thermal oxide).
The control circuits of a matrix of memory cells and the other data-processing circuits, generally known as peripheral circuits, which may be integrated with the memory on the same silicon die, contain components, such as MOSFET transistors and capacitors, which have structural elements similar to those of the memory cells.
In particular, the bodies of the transistors and one electrode of each capacitor of the peripheral circuits are constituted by the silicon substrate, the gate electrodes of the transistors and the other electrode of each capacitor being constituted by an electrically-conductive element of metal or doped polycrystalline silicon.
Although the components of the first type, that is, the memory cells, and the components of the second type, that is, the transistors and the capacitors of the peripheral circuits, are structurally similar, it is not always possible or convenient to produce them by exactly the same operations. In particular, the thermal oxide used as the dielectric in the memory cells and in the peripheral circuits has to have different characteristics according to the function it performs in the two types of components. In fact, whereas in the transistors and in the capacitors of the peripheral circuits maximum electrical insulation between the electrodes separated by the dielectric is of essential importance, in the cells it is important for the oxide to be able to withstand movements of electrical charges within it, without deterioration, during the processes which occur during writing and during any erasure from the cells. Rapid degradation of the thermal oxide in the end results in a shorter operative life of the memory.